Embodiments of the present invention relate to a method of fabricating thin film batteries on a substrate which uses a cutting process.
Thin film batteries are used in various applications, such as portable electronics, medical devices and space systems. A thin film battery typically comprises a substrate having one or more battery component films that include an electrolyte sandwiched between electrode films such an anode, cathode, and/or current collector films, that cooperate to store electrical charge and generate a voltage. The battery component films that are typically less than 100 microns allowing the thin film batteries to be less than about 1/100th of the thickness of conventional batteries. The battery component films are formed by processes, such as for example, physical and chemical vapor deposition (PVD or CVD), oxidation, nitridation, and electroplating.
Furthermore, in many applications, thin film batteries having thin or compact dimensions are desirable and the energy density and specific energy of the battery are also important performance measures. The energy density level is the fully charged output energy level per unit volume of the battery. The specific energy level is the fully charged output energy level per unit weight of the battery. However, conventional battery films and substrate materials often constrain the size dimensions, and limit the maximum energy density and specific energy levels that can be obtained from such batteries.
Battery performance can be improved by forming the battery on thin plate-like substrates, such as for example ceramic substrates composed of Al2O3 or SiO2, which increase the energy to volume/weight ratio of the battery. In such processes, an array of battery cells is formed on the plate-like substrate, and thereafter, individual battery cells are mechanically cut out from the substrate. As one example, the battery cells can be cut out with a diamond or carbide cutting wheel. However, the battery cells are often damaged due to cracking along the edges of the cut. Micro-cracks that originate from the fracture points along the cutting line can also affect the performance of the thin film battery cells and result in cell failure. Increasing the width along the cutting edge to provide a wider gap or spacing between the battery cells is undesirable because it decreases the energy density of the final battery cells and also reduces substrate yields per unit area. The cutting process can also contaminate the battery cells with the cutting or grinding residue. Further, handling of the thin plate-like substrates with micron sized battery films is difficult during the cutting process because some battery component films, such as for example, lithium or other films, are adversely affected when exposed to air or moisture. Thus, for a number of reasons, conventional battery cutting processes are often inadequate and result in low battery cell yields.
Cutting of the battery cells is even more problematic when the battery cells are built on very crystalline substrates having cleavage places. For example, mica substrates have been used to reduce the total weight and volume of the battery while providing good mechanical strength for the battery cells and dielectric strength. Mica has a flat planar structure with cleavage properties that allow mica to be split into thin foils along its cleavage planes. Thus, the mica substrate can be made very thin with thicknesses of less than about 100 microns or even less than about 25 microns. However, it is difficult to cut a substrate comprising a mica sheet because the substrate can split along the cleavage planes while it is being cut. Thus, cutting of sheet-like substrates with cleavage planes generates special cutting problems.
Thus it is desirable to be able to cut a substrate to form individual battery cells without damaging the cells. It is also desirable to be able to cut a battery substrate composed of mica without causing cleavage faults along the cutting line. It is further desirable not to contaminate the thin films that form the battery cells with grinding or cutting residue. It is also desirable to reduce oxidation of battery cells during processing by their exposure to the external environment.